[HBR] Another HBR Project

[hbr@mailman.qth.net]

In a message dated 8/25/03 8:24:40 PM Eastern Daylight Time, 
waltah@earthlink.net writes:


> I want to use a 
> 12BH7 (dual medium mu triode) for p-p audio output, driving it with a 
> phase splitter; there may be trouble getting enough drive voltage.  
> 

I've used a p-p 12BH7 as a "speaker driver" for the past several years now. 
Transformer coupled on the input. Lots of clean audio - much cleaner than 
typical audio pentodes without negative feedback.

Output iron trick: Use a small power transformer instead of a "real" audio 
trans. Core saturation not nearly as much a problem in pushpull. Filament 
windings in series will be a decent voicecoil match. Watch phasing, though!

Example: 520 VCT power transformer to 5 and 6.3 V heater windings in series. 
520/11.3 is 46-to-1 voltage ratio or 2117-to-1 impedance ratio. So a 4 ohm 
voice coil will present an impedance of about 8470 ohms to the pushpull plates. 

73 de Jim, N2EY


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